The present invention provides a process and an apparatus for enhancing the imaging of subtle structures, such as tumor tissue within a soft tissue matrix. Specifically, the process and apparatus provides a transmissive ultrasonic holography imaging system having an acoustical opaque small element variably placed so as to block the contribution to the image by sound energy transmitted through the object but not scattered by the object being imaged. The present invention further provides a process and apparatus for a transmissive ultrasonic holography imaging system comprising an acoustical opaque planar element having an opening so as to pass unscattered ultrasonic energy (i.e., sound) but to block the contribution to the image by ultrasonic energy that is transmitted through the object and scattered by the object. The present invention further provides an alternate process and apparatus which provides for an acoustical planar element variably placed so as to block all or substantially all of the ultrasonic energy transmitted through the object except that scattered from a selected volume within the object being imaged. The process and apparatus further provides that these two separate image contributions are used and analyzed separately or combined for improved diagnosis of subtle structures. One of the results of utilizing the inventive process provides for improved imaging visualization of subtle objects by providing a means of imaging only with sound scatter from subtle objects because only ultrasound that interferes with the object is transmitted to a holographic detector and reconstructed within the detector. More specifically, the invention provides a process to separately use only specific portions of the transmitted sound wave to make separate images of the object and utilize a combination of such images to provide greater detailed information about subtle structures within the object.
The central element field of holography is fulfilled by combining or interfering an object wave or energy with a reference wave or energy to form an interference pattern referred to as the hologram. A fundamental requirement for the forming of the hologram and the practice of holography is that the initial source of the object wave and reference wave or energy are coherent with respect to the other wave. That is to say, that all parts of both the object wave and the reference wave are of the same frequency and of a defined orientation (a fixed spatial position and angle between the direction of propagation of the two sources). When performing holography the object wave is modified by interference with structure within the object of interest. As this object wave interacts with points of the object the three-dimensional features of the object impart identifying phase and amplitude changes on the object wave. Since the reference wave is an unperturbed (pure) coherent wave, its interference with the object wave results in an interference pattern which identifies the 3-D positioning and characteristics (ultrasonic absorption, diffraction, reflection, and refraction) of the scattering points of the object.
A second process, (the reconstruction of the hologram) is then performed when a coherent viewing source (usually light from a laser) is transmitted through or reflected from the hologram. The hologram pattern diffracts light from this coherent viewing or reconstructing source in a manner to faithfully represent the 3-D nature of the object, as seen by the ultrasonic object wave.
To reiterate, to perform holography coherent wave sources are required. This requirement currently limits practical applications of the practice of holography to the light domain (e.g., a laser light) or the domain of acoustics (sometimes referred to as ultrasound due to the practical application at ultrasonic frequencies) as these two sources are currently the only available coherent energy sources. Thus, further references to holography or imaging system will refer to the through transmission holographic imaging process that uses acoustical energies usually in the ultrasonic frequency range. In the practice of ultrasound holography, one key element is the source of the ultrasound, such as a large area coherent ultrasound transducer. A second key element is the projection of the object wave from a volume within the object (the ultrasonic lens projection system) and a third is the detector and reconstruction of the ultrasonic hologram into visual or useful format.
Although other configurations can be utilized, a common requirement of the source transducers for both the object and reference waves is to produce a large area plane wave having constant amplitude across the wave front and having a constant frequency for a sufficient number of cycles to establish coherence. Such transducers will produce this desired wave if the amplitude of the ultrasound output decreases in a Gaussian distribution profile as the edge of the large area transducer is approached. This decreasing of amplitude reduces or eliminates the xe2x80x9cedge effectxe2x80x9d from the transducer edge, which would otherwise cause varying amplitude across the wave front as a function distance from the transducer.
In the process of through transmission ultrasonic holographic imaging, the pulse from the object transducer progresses through the object, then through the focusing lens and at the appropriate time, the pulse of ultrasound is generated from the reference transducer such that the object wave and reference wave arrive at the detector at the same time to create a interference pattern (the hologram). For broad applications, the transducers need to be able to operate at a spectrum or bandwidth of discrete frequencies. Multiple frequencies allow comparisons and integration of holograms taken at selected frequencies to provide an improved image of the subtle changes within the object.
A hologram can also be formed by directing the object wave through the object at different angles to the central axis of the lens means. This is provided by either positioning or rotating the object transducer around the central axis of the lens means by using multiple transducers positioned such that the path of transmission of the sound is at an angle with respect to the central axis of the lens means.
With a through-transmission imaging system, it is important to determine the amount of resolution in the xe2x80x9czxe2x80x9d dimension that is desirable and achievable. Since the holographic process operates without limits of mechanical or electronic devices but rather reconstructs images from wave interactions, the resolution achievable can approach the theoretical limit of xc2xd the wavelength of the ultrasound used. However, it may be desirable to limit the xe2x80x9czxe2x80x9d direction image volume so that one can xe2x80x9cfocusxe2x80x9d in on one thin volume slice. Otherwise, the amount of information may be too great. Thus, it is of value to develop a means for projecting a planar slice within a volume into the detector plane. One such means is a large aperture ultrasonic lens system that will allow the imaging system to xe2x80x9cfocusxe2x80x9d on a plane within the object. Additionally, this lens system and the corresponding motorized, computer controlled lens drive will allow one to adjust the focal plane and at any given plane to be able to magnify or demagnify at that z dimension position.
The image is detected and reconstructed at the detector. Standard photographic film may be used for the recording of light holograms and the 3-D image reconstructed by passing laser light through the film or reflecting it from the hologram pattern embossed on the surface of an optical reflective surface and reconstructing the image by reflecting light from the surface. However, there is no equivalent xe2x80x9cfilmxe2x80x9d material to record the intricate phase and amplitude pattern of a complex ultrasonic wave. One of the most common detectors uses a liquid-air surface or interface to record, in a dynamic way, the ultrasonic hologram formed. The sound energy at the frequency of ultrasound (above range of human hearing) will propagate with little attenuation through a liquid (such as water) but cannot propagate through air. At these higher frequencies (e.g., above 1 MHz) the ultrasound will not propagate through air because the wavelength of the sound energy is so short (xcex(wavelength)=v(velocity)/ƒ(frequency)). The density of air (approximately 0.00116 g/cm3) is not sufficient to couple these short wavelengths and allow them to propagate. On the other hand the density of a liquid (e.g., water) is a favorable media to couple and propagate such sound. For example, the velocity of sound in air is approximately 346 meters/second whereas in water it is approximately 1497 meter/second. Thus, for water, both the density (1 g/cm3) and the wavelength (xcx9c1.5 mm at 1 MHz) are significantly large such that ultrasound can propagate with little attenuation. Whereas, for air both the density (0.00116 g/cm3) and wavelength (0.346 mm at 1 MHz) are sufficiently small such that the energy at these ultrasonic frequencies will not propagate.
Thus, when ultrasound propagating in a liquid encounters a liquid-air interface the entire amount of the energy is reflected back into the liquid. Since ultrasound (or sound) propagates as a mechanical force it is apparent that the reflection (or changing direction of propagation) will impart a forward force on this liquid air interface. This force, in turn, will distort the surface of the liquid. The amount of surface distortion will depend upon the amplitude of the ultrasound wave at each point being reflected and the surface tension of the liquid. Thus, the pattern of the deformation is the pattern of the phase and amplitude of the ultrasonic wave.
It is in this manner that a liquid-air interface can be commonly used to provide a near realtime recorder (xe2x80x9cfilm equivalentxe2x80x9d) for an ultrasonic hologram. The shape of the surface deformation on this liquid-air detector is the representation of the phase and amplitude of the ultrasonic hologram formed by the interference of the object and reference ultrasonic waves.
The greatest value of the ultrasonic holographic process is achieved by reconstructing the hologram in a usable manner, usually in light, to make visible the structural nature of the initial object. In the case of a liquid-air interface, the reconstruction to achieve the visible image is accomplished by reflecting a coherent light from this liquid-air surface. This is the equivalent process to reflecting laser light from optically generated hologram that is embossed on the surface of a reflecting material (e.g., thin aluminum film).
The reflected light is diffracted (scattered) by the hologram to diffracted orders, each of which contains image information about the object. These diffracted orders are referred to as xc2x1n th orders. That part of the reconstructing light that does not react with the hologram is referred to as zero order and is usually blocked so that the weaker diffracted orders can be imaged. The higher the diffracted order the greater the separation angle from the zero order of reflected light.
Once reconstructed, the image may be viewed directly, by means of a video camera or through post processing processes.
Ultrasonic holography as typically practiced is illustrated in FIG. 1. A plane wave of sound (1a) (ultrasound) is generated by the object (large area) transducer (1) (U.S. Pat. No. 5,329,202). The sound is scattered (diffracted) by structural points within the object. The scattered sound (2a) is from the internal object points that lie in the focal plane (2) are focused (projected) into the ultrasonic hologram plane (6). The focusing takes place by use of ultrasonic lens (3) (U.S. Pat. No. 5,235,553) which focuses the scattered sound into a hologram detector surface (6) and the unscattered sound into a point (4). The lens means also allows the imaging process to magnify the image or change focus position (U.S. Pat. No. 5,212,571). Since the focus point of the unscattered sound (4) is prior to the holographic detector plane (6), this portion of the total sound again expands to form the transparent image contribution (that portion of the sound that transmitted through the object as if it were transparent or semitransparent). In such an application, an ultrasound reflector (5) is generally used to direct the object sound at a different angle (preferably vertically to allow for the holographic detector to have a surface parallel to ground to avoid gravity effects), thus impinging on the horizontal detector plane usually containing a liquid which is deformed by the ultrasound reflecting from the liquid-air interface. When the reference wave (7) and the object wave are simultaneous reflected from this detector, the deformation of the liquid-air interface is the exact pattern of the ultrasonic hologram formed by the object wave (1a combined with 2a) and the xe2x80x9coff-axisxe2x80x9d reference wave (7).
This ultrasonic hologram formed in the holographic detector (6) is subsequently reconstructed for viewing by using a coherent light source (9), which may be passed through an optical lens (8), and reflected from the holographic detector surface (U.S. Pat. No. 5,179,455). This reflected coherent light contains two components. These are A: The light that is reflected from the ultrasound hologram which was not diffracted by the ultrasonic holographic pattern which is focused at position (10) and referred to as undiffracted or zero order light; and B: The light that does get diffracted from/by the ultrasonic hologram is reflected at an xe2x80x9coff-axisxe2x80x9d angle from the zero order at position (11) and referred to as the xe2x80x9cfirst orderxe2x80x9d image view when passed through a spatial filter (12). It is noted that this reconstruction method produces multiple diffraction orders each containing the ultrasonic object information. Note also both + and xe2x88x92 multiple orders of the diffracted image are present and can be used individually or in combinations to view the optical reconstructed image from the ultrasonically formed hologram by modifying the spatial filter (12) accordingly.
Therefore, there is a need in the art to improve image quality by recognizing and utilizing the effects of diffraction generated by internal structures within the object. This need is particularly strong for breast cancer screening techniques that now utilize invasive mammography (providing the patient with a dose of radiation from XRay imaging) and yet do not have high quality images that lend a sense of three dimensional structure to breast tissue.
That portion of the ultrasound wave that passes through the imaged object without interference with the object can be a major contributor in xe2x80x9csemitransparent objectsxe2x80x9d (that is, an object that scatters a small portion of the sound waves directed at the object). Since many objects of interest can be rather transparent to sound, (e.g. human soft tissue normal structures and tumor tissue of solid tumors) there is formed a bright and strong white light contribution to the image from this sound that does not interfere with the object. When one wants to detect and determine the characteristic of subtle changes in the object (e.g., determining tissue characteristics) this background bright image contribution can overpower the resolution of small and subtle contributions of tissue change. Therefore, there is a need in the art to improve resolution characteristics of transmissive ultrasonic imaging so as to be able to distinguish subtle differences within the object (i e., so as to be able to image tumor tissue within surrounding soft breast tissue).
The present invention is based upon the surprising discovery for transmission ultrasonic holography imagining that an acoustical opaque small element variably placed after the lens or between multiple lens means so as to block sound that is transmitted through the object but which is not scattered by the object (referred to as xe2x80x9cunscatteredxe2x80x9d sound or xe2x80x9cunscatteredxe2x80x9d ultrasonic energy or undiffracted sound), such that only sound scattered from points within the object may be used to provide an image results in unique qualities and information.
The present invention provides an apparatus for imaging subtle structures internal to an object, comprising:
(a) one or a plurality of ultrasonic transducers directing unscattered ultrasonic energy in the form of a wave toward an object to be imaged;
(b) an acoustic lens means for focusing the unscattered ultrasonic energy to a focal point downstream of a first lens and having a centerline, wherein the lens means comprises one or a plurality of lenses, wherein the focal point is location at which the unscattered ultrasonic energy is focused; and wherein the acoustic lens means focuses ultrasonic energy scattered from structures within the object;
(c) an acoustically opaque element selectively positioned at the focal point, aligned perpendicular to the axis of the lens means, whereby the acoustically opaque element either (i) prevents transmission of ultrasonic energy directed to the focal point and allows passage of scattered ultrasonic energy not directed to the focal point, or (ii) allows only passage of unscattered ultrasonic energy, or (iii) allows only passage of ultrasonic energy that is scattered from a selected volume within the object; and
(d) a holographic detector having a surface aligned perpendicular to the centerline of the acoustic lens means.
Preferably, the acoustically opaque element of (i) that allows passage of scattered ultrasonic energy comprises a small solid mass of acoustically opaque material. Preferably, the acoustically opaque element of (ii) that allows passage of unscattered ultrasonic energy comprises a planar shaped object of acoustically opaque material having an opening such that ultrasonic energy directed to the focal point passes through the opening to prevent transmission of ultrasonic energy scattered from the object but allowing passage of ultrasonic energy directed to the focal point. Preferably, the acoustically opaque element of (iii) allows passage of ultrasonic energy that is scattered from a selected volume within the object comprises a concentric circular shaped planar object having a center hole and alternating opening between rings of acoustically opaque material. Preferably, the wave of ultrasonic energy generated by the transducer with a contour shape is selected from the group consisting of planar, cylindrical, spherical, and combinations thereof. Most preferably, the wave of ultrasonic energy generated by the transducer is focused by the acoustic lens at a position along the path of transmission before reaching the hologram detector surface. Most preferably, the acoustic lens means focuses the unscattered wave of ultrasonic energy to a focal point prior to the detector and any generated diffraction waves generated within the object at the hologram detector surface. Preferably, the apparatus further comprises a reflective means to direct the waves of ultrasonic energy to a vertical orientation. Preferably, the acoustically opaque material has entrapped voids or air. Most preferably, the acoustical opaque material is selected from the group consisting of cork, porous polymers, open or closed cell foams, and combinations thereof.
The present invention further provides a process for improved imaging of interior structures of an object, comprising:
(a) providing ultrasonic energy to transmit through the object to form unscattered transmitted ultrasonic energy not scattered by the object and scattered ultrasonic energy, which is scattered by the object;
(b) focusing the unscattered ultrasonic energy to a focal point with an acoustic lens means having a centerline, wherein the unscattered ultrasonic energy is focused to a point downstream of lens means and prior to a detector means, and wherein scattered ultrasonic energy is focused to a plane corresponding to a plane of the detector means;
(c) providing a solid acoustically opaque element made from acoustically opaque material selectively positioned at the focal point of the unscattered ultrasonic energy to prevent transmission of unscattered ultrasonic energy or a planar-shaped acoustically opaque element having a hole positioned at the focal point and made from acoustically opaque material to prevent transmission of scattered ultrasonic energy; and
(d) imaging the interior structures of the object with a holographic detector means having a surface aligned perpendicular to the direction of gravity by recording with a first mode image created by the unscattered ultrasonic energy and a second mode image/ created by the scattered ultrasonic energy, then comparing images of the first mode and the second mode.
Preferably, the scattered ultrasonic energy carries spatial phase and amplitude information corresponding to the three dimensional nature of the object""s interior structure. Preferably, the unscattered ultrasonic energy carries information corresponding to the acoustical reflection and absorption characteristics of the object. Preferably, the wave of unscattered ultrasonic energy is focused by the lens means prior to reaching the plane of the hologram detector surface. Most preferably, the acoustic lens means focuses the unscattered ultrasonic energy in the form of an object wave to a focal point prior to the plane of the detector surface and any generated scattered ultrasonic energy in the form of diffraction waves generated within the object to the plane of the detector surface. Preferably, the acoustically opaque element is in the form of a plane having an opening. Preferably, the acoustically opaque material having entrapped voids or air. Most preferably, the acoustical opaque material is selected from the group consisting of cork, porous polymers, open or closed cell foams, and combinations thereof. Preferably, the unscattered ultrasonic energy in the form of a wave is focused to the focal point of the lens means by adjusting the lens means along a z-axis with an electromechanical means. Most preferably, the electromechanical means for adjusting the lens means is controlled by a computer adjusting both the lens means to form a focal point and positioning the acoustically opaque element to the focal point of the unscattered ultrasonic energy wave.
The present invention further provides a process for improved imaging of interior structures of an object, comprising:
(a) providing ultrasonic energy to transmit through the object to form unscattered transmitted ultrasonic energy not scattered by the object and scattered ultrasonic energy;
(b) focusing the unscattered ultrasonic energy to a focal point with an acoustic lens means having a centerline, wherein the unscattered ultrasonic energy is focused to a point downstream of lens means and prior to a detector means, and wherein scattered ultrasonic energy is focused to a plane corresponding to a plane of the detector means;
(c) providing an acoustically opaque element made from acoustically opaque material -selectively positioned between the object and detector, wherein the acoustically opaque element is planar and circular wherein there is a center hole and concentric circular ribbons of acoustically opaque material; and
(d) imaging the interior structures of the object with a holographic detector means having a surface aligned perpendicular to the centerline of the acoustic lens means.
Preferably, the scattered ultrasonic energy carries spatial phase and amplitude information corresponding to the three dimensional nature of the object""s interior structure. Preferably, the unscattered ultrasonic energy carries information corresponding to the acoustical reflection and absorption characteristics of the object. Most preferably, the acoustic lens means focuses the unscattered ultrasonic energy in the form of an object wave to a focal point prior to the plane of the detector surface and any generated scattered ultrasonic energy in the form of diffraction waves generated within the object to the plane of the detector surface. Preferably, the acoustically opaque material comprises entrapped voids or air. Most preferably, the acoustical opaque material is selected from the group consisting of cork, porous polymers, open or closed cell foams, and combinations thereof. Preferably, the unscattered ultrasonic energy in the form of a wave is focused to the focal point of the lens means by adjusting the lens means along a z-axis with an electromechanical means. Most preferably, the electromechanical means for adjusting the lens means is controlled by a computer adjusting both the lens means to form a focal point and positioning the acoustically opaque element to the focal point of the unscattered ultrasonic energy wave.